A sample is irradiated with an electron beam while being scanned therewith to detect secondary electrons emitted from the sample, thereby being capable of observing a structure of a sample surface. This is called a scanning electron microscope (hereinafter abbreviated as “SEM”). On the other hand, even when the sample is irradiated with the ion beam while being scanned therewith to detect charged particles such as the secondary electrons, secondary ions, or reflected ions emitted from the sample, the structure of the sample surface can be observed. This is called a canning ion microscope (hereinafter abbreviated as “SIM”).
The ion beam has a characteristic of being sensitive to information on the sample surface as compared with the electron beam. This is because an excitation area of the secondary electrons is localized on the sample surface more than the irradiation of the electron beam. In the electron beam, aberration is generated due to a diffraction effect because a property as waves of the electrons cannot be ignored. On the other hand, because the ion beam is heavier than the electrons, the aberration caused by the diffraction effect is extremely small as compared with the electrons. In particular, when a gas field ion source with high luminance is used, the ion beam can be focused extremely finely as compared with the electron beam. In other words, ultrahigh resolution of the sample surface is enabled.
Meanwhile, the gas field ion source is supplied with a gas such as helium to a metal emitter tip having a tip curvature radius of about 100 nm, and a high voltage of several kV or more is applied to the emitter tip, to thereby field ionize gas molecules, and draw the ionized gas as an ion beam. The feature of the ion source resides in that an extremely fine ion beam can be generated because an ion energy width is narrow and a size of an ion generation source is small. In addition, in order to increase an ion radiation angle current density of the gas field ion source, the emitter tip is cooled to an extremely low temperature and a pressure of the ionized gas around the emitter tip is set to, for example, about 10−2 to several Pa.
Further, when a miniaturized semiconductor sample is irradiated with the extremely fine ion beam so as to be scanned therewith to detect the detected secondary electrons, a dimension of the surface structure of the semiconductor sample can be detected with high accuracy. Further, when a membrane sample is irradiated with the extremely fine ion beam and ions that have been transmitted through the sample are detected, information reflecting an internal structure of the sample can be obtained.
Patent Literature 1 discloses a charged particle microscope including a vacuum chamber, a first pump that exhausts a gas in the vacuum chamber, an emitter tip that is disposed in the vacuum chamber, an extraction electrode that is disposed to face the emitter tip, and gas supply means that supplies the gas to the emitter tip. In the charged particle microscope, the ionized gas supply means includes a second pump that circulates the gas that has not been used in the emitter tip, and the second pump contains a gas adsorbing material that adsorbs the ionized gas.
In addition, Patent Literature 2 discloses the use of hydrogen or helium in a gas field ion source, and the use of a mixed gas of hydrogen and helium or a mixed gas with other gases.